Coated inserts for rough milling

ABSTRACT

Coated milling insert has a WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiC x N y  with columnar grains followed by a layer of κ-Al 2 O 3  and a top layer of TiN. The coated milling insert is particularly useful for milling of grey cast iron with or without cast skin under wet conditions at low and moderate cutting speeds and milling of nodular cast iron and compacted graphite iron with or without cast skin under wet conditions at moderate cutting speeds.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,767,583 B2 filedon Aug. 13, 2003, which is a divisional application of U.S. Pat. No.6,638,609 B2, filed on Oct. 29, 2001, which claims the benefit ofpriority to Swedish Application No. 0004079 - 0 filed Nov. 8, 2000.

This application is a divisional of application Ser. No. 09/984,145,filed on Oct. 29, 2001, now U.S. Pat. No. 6,638,609.

This application claims priority under 35 U.S.C. §§119 and/or 365 toApplication No. 004079-0 filed in Sweden on Nov. 8, 2001, the entirecontent of which is hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to coated cemented carbide cutting toolinserts, particularly useful for milling of grey cast under wetconditions, preferably at low and moderate cutting speeds but also formilling of nodular cast iron and compacted graphite iron under wetconditions at moderate cutting speeds.

2. Background of the Invention

It is well known that for cemented carbide cutting tool inserts used inthe machining of cast irons, the cutting edge is worn by different wearmechanisms such as chemical and abrasive wear but the cutting edge isgenerally also subjected to crack formation due to the intermittentcutting load, resulting in so called chippings and edge fractures causedby different types of cracks in the inserts.

Different types of crack patterns may appear during machining of castirons. One important type is the so called comb cracks, which are formedperpendicularly to the cutting edge. The formation of comb cracks isstrongly influenced by the cooling conditions during cutting. Inparticular, the use of fluid coolant increases the tendency to form combcracks, often also called thermal cracks. The use of fluid coolant leadsto large temperature gradients and thermal tensile stresses in theinsert surface, increasing the tendency for formation of surface cracks,in particular in the case of coated cutting tool inserts where the hardbut brittle ceramic surface coating is prone to crack under conditionsinvolving unfavourable thermal tensile stresses. Cracks in the coatingincreases the risk for chipping and edge fractures and for flaking ofthe coating.

Characteristic for cast irons is the so called surface skin, the surfacezone of the cast component often contains a structure which deviatesconsiderably from the bulk structure and also contains hard inclusionand sand from the mould. In this case, a coated cemented carbide insertmust be used including a substrate with the proper toughness of thecemented carbide grade and on the surface a wear resistant refractorycoating.

Furthermore, different cutting conditions such as cutting speed, depthof cut, cutting feed rate and also external factors such as vibrationsof the work piece and the above mentioned surface zone in iron casting,etc., require a plurality of different properties of the cutting edge.

Commercial cemented carbide tool inserts for milling of cast irons underwet conditions are usually optimised with respect to one or two of thewear types observed.

U.S. Pat. No. 5,912,051 discloses a coated cutting insert particularlyuseful for dry milling of grey cast iron.

U.S. Pat. No. 5,863,640 discloses a coated turning insert particularlyuseful for intermittent turning in low alloyed steel.

In U.S. Pat. No. 6,062,776 is disclosed a coated cemented carbidecutting tool particularly designed for the wet and dry milling ofworkpieces of low and medium alloyed steels or stainless steels, with orwithout abrasive surface zones, in machining operations requiring a highdegree of toughness of the carbide cutting edge. The external cuttingconditions are characterised by complex shapes of the workpiece,vibrations, chip hammering, recutting of the chips etc.

In U.S. Pat. No. 6,177,178 is disclosed a coated cemented carbidecutting tool particularly designed for the wet and dry milling of lowand medium alloyed steels.

WO 01/16388 discloses a coated insert particularly useful for milling inlow and medium alloyed steels with or without abrasive surface zonesduring dry or wet conditions at high cutting speed, and milling hardenedsteels at high cutting speed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It has now surprisingly been found that by combining many differentfeatures cutting tool inserts, preferably for milling, can be obtainedwith excellent cutting performance when milling grey cast iron usingfluid coolant at low and moderate cutting speeds as well as in millingof nodular and compacted graphite iron using fluid coolant at moderatecutting speeds, in iron castings with or without cast skin.

The cutting tool inserts according to the invention show improvedproperties with respect to the different wear types prevailing at thesecutting conditions as earlier mentioned.

The cutting tool inserts according to the invention consist of: acemented carbide body with a relatively high W-alloyed binder phase andwith a well balanced chemical composition and grain size of the WC, acolumnar TiC_(x)N_(y)-layer, a κ-Al₂O₃-layer, a TiN-layer and optionallyfollowed by smoothening the cutting edges by brushing the edges.

According to the present invention coated cutting tool inserts areprovided consisting of a cemented carbide body with a composition of7.3-7.9 wt. % Co, preferably 7.6 wt. % Co, 1.0-1.8 wt. % cubic carbides,preferably 1.4-1.7 wt. % cubic carbides of the metals Ta and Nb andbalance WC. The average grain size of the WC is in the range of about1.5-2.5 μm, preferably about 1.8 μm.

The cobalt binder phase is rather highly alloyed with W. The content ofW in the binder phase can be expressed as the CW-ratio:CW-ratio=Ms/(wt % Co ×0.0161)where Ms is the saturation magnetization of the cemented carbide body inkA/m hAm² /kg and wt. % Co is the weight percentage of Co in thecemented carbide. The CW-value is a function of the W content in the Cobinder phase. A high CW-value corresponds to a low W-content in thebinder phase.

It has now been found according to the present invention that improvedcutting performance is achieved if the cemented carbide body has aCW-ratio of 0.86-0.94. The cemented carbide may contain small amounts,<3 vol. %, of η-phase (M₆C), without any detrimental effect.

The coating comprises

-   -   a first (innermost) layer of TiC_(x)N_(y)O_(z) with x+y+z=1, y>x        and z<0.2, preferably y>0.8 and z=0, with equiaxed grains with        size <0.5 μm and a total thickness <1.5 μm preferably >0.1 μm.    -   a layer of TiC_(x)N_(y) with x+y=1, x>0.3 and y>0.3, preferably        x≧0.5, with a thickness of 1-4 μm, preferably 2-2.7 μm, with        columnar grains and with an average diameter of <5 μm,        preferably 0.1-2 μm.    -   a layer of smooth, fine-grained (grain size about 0.5-2 μm)        Al₂O₃ consisting essentially of the κ-phase. However, the layer        may contain small amounts (<5 vol. %) of other phases such as η-        or the α-phase as determined by XRD-measurement. The Al₂O₃-layer        has a thickness of 1-2.5 μm, preferably 1.2-1.7 μm.    -   a further 0.5-1.0 μm thick layer of TiN. This outermost layer of        TiN has a surface roughness Rmax≦0.4 μm over a length of 10 μm.        The TiN-layer is preferably removed along the cutting edge and        the underlying alumina layer may be partly or completely removed        along the cutting edge.

The present invention also relates to a method of making coated cuttingtool inserts consisting of a cemented carbide body with a composition of7.3-7.9 wt. % Co, preferably 7.6 wt. % Co, 1.0-1.8 wt. % cubic carbides,preferably 1.4-1.7 wt. % cubic carbides of the metals Ta and Nb andbalance WC. The average grain size of the WC is in the range of about1.5-2.5 μm, preferably about 1.8 μm. Onto the cemented carbide body isdeposited.

-   -   a first (innermost) layer of TiC_(x)N_(y)O_(z) with x+y+z=1, y>x        and z<0.2, preferably y>0.8 and z=0, with equiaxed grains with        size <0.5 μm and a total thickness <1.5 μm preferably >0.1 μm        using known CVD-methods.    -   a layer of TiC_(x)N_(y) with x+y=1, x>0.3 and y>0.3, preferably        x≧0.5, with a thickness of 1-4 μm, preferably 2-2.7 μm, with        columnar grains and with an average diameter of <5 μm,        preferably 0.1-2 μm using preferably MTCVD-technique (using        acetonitrile as the carbon and nitrogen source for forming the        layer in the temperature range of 700-900° C.). The exact        conditions, however, depend to a certain extent on the design of        the equipment used.    -   a smooth Al₂O₃-layer essentially consisting of κ-Al₂O₃ is        deposited under conditions disclosed in e.g. U.S. Pat. No.        5,674,564. The Al₂O₃ layer has a thickness of 1-2.5 μm,        preferably 1.2-1.7 μm.    -   a 0.5-1.0 μm thick layer of TiN with a surface roughness        Rmax≦0.4 μm over a length of 10 μm.

The smooth coating surface is obtained by a gentle wet-blasting thecoating surface with fine grained (400-150 mesh) alumina powder or bybrushing the edges with brushes based on e.g. SiC as disclosed e.g. inU.S. Pat. No. 5,861,210. The TiN-layer is preferably removed along thecutting edge and the underlying alumina layer may be partly orcompletely removed along the cutting edge.

The invention also relates to the use of cutting tool inserts accordingto above for wet milling using fluid coolant of cast irons such as greycast iron, compacted graphite iron and nodular iron particularly greycast iron at a cutting speed of 70-180 m/min and a feed of 0.1-0.4μm/tooth depending on cutting speed and insert geometry.

EXAMPLE 1

A. Cemented carbide milling inserts in accordance with the inventionwith the composition 7.6 wt. % Co, 1.25 wt. % TaC, 0.30 wt. % NbC andbalance WC with average grain size of 1.8 μm, with a binder phasealloyed with W corresponding to a CW-ratio of 0.87 were coated with a0.5 μm equiaxed TiC_(0.05)N_(0.95)-layer (with a high nitrogen contentcorresponding to an estimated C/N-ratio of 0.05) followed by a 2.6 μmthick TiC_(0.54)N_(0.46)-layer, with columnar grains by usingMTCVD-technique (temperature 850-885° C. and CH₃CN as thecarbon/nitrogen source). In subsequent steps during the same coatingcycle, a 1.3 μm thick layer of Al₂O₃ was deposited using a temperature970° C. and a concentration of H₂S dopant of 0.4% as disclosed in U.S.Pat. No. 5,674,564. A thin (0.5 μm) layer of TiN was deposited on topaccording to known CVD-technique. XRD-measurement showed that theAl₂O₃-layer consisted of 100% κ-phase.

The coated inserts were brushed using a nylon straw brush containing SiCgrains. Examination of the brushed inserts in a light optical microscoperevealed that the outermost, thin TiN-layer and some of the Al₂O₃-layerhad been brushed away along the very cutting edge, leaving there asmooth Al₂O₃-surface. Coating thickness measurements on cross sectioned,brushed inserts showed that the outermost TiN-layer and roughly half theAl₂O₃-layer had been removed along the edge line.

B. Commercial cemented carbide milling inserts with the composition 9wt. % Co, 1.23 wt. % TaC, 0.30 wt. % NbC and balance WC with a WC grainsize in average of 1.7 μm, with a binder phase alloyed with Wcorresponding to a CW-ratio of 0.92 were coated with an innermost 0.5 μmequiaxed TiN-layer followed by a 5.5 μm thick Ti(C,N)-layer, withcolumnar grains by using MTCVD-technique and outermost a 4 μm thicklayer of Al₂O₃. XRD-measurement showed that the Al₂O₃-layer consisted of100% α-phase.

C. Cemented carbide milling inserts with the composition 6 wt. % Co andbalance WC with average grain size 1.8 μm, with a binder phase alloyedwith W corresponding to a CW-ratio of 0.90 were coated with a 2 μm thickTiC-layer using known CVD-technique. In subsequent steps during the samecoating cycle, a 1 μm thick layer of Al₂O₃ was deposited.

Inserts from A, B and C were tested in face milling of grey cast ironcylinder heads.

Operation: Face milling - roughing Work-piece: Cylinder head Material:Peailitic grey cast iron, alloyed, Cutting speed: 116 in/min Feedrate/tooth: 0.32 μm/rev. Depth of cut: 2 μm Insert-style: TNEF 1204AN-CANote: Wet, single tooth milling Results: Tool-life, number of passes peredge Grade A: (invention) 99 Grade B: (prior art) 60 Grade C: (priorart) 49 Tool-life criterion was chippings and fractures of the edges.

EXAMPLE 2

D. Cemented carbide milling inserts in accordance with the inventionwith the composition 7.6 wt. % Co, 1.25 wt. % TaC, 0.30 wt. % NbC andbalance WC with an average grain size of 1.75 μm, with a binder phasealloyed with W corresponding to a CW-ratio of 0.88 were coated with a0.5 μm equiaxed TiC_(0.05)N_(0.095)-layer (with a high nitrogen contentcorresponding to an estimated C/N-ratio of 0.05) followed by a 2.0 μmthick TiC_(0.54)N₄₆-layer, with columnar grains by using MTCVD-technique(temperature 850-885° C. and CH₃CN as the carbon/nitrogen source). Insubsequent steps during the same coating cycle, a 1.4 μm thick layer ofAl₂O₃ was deposited using a temperature 970° C. and a concentration ofH₂S dopant of 0.4% as disclosed in U.S. Pat. No. 5,674,564. A thin (0.5μm) layer of TiN was deposited on top according to known CVD-technique.XRD-measurement showed that the Al₂O₃-layer consisted of 100% κ-phase.

The coated inserts were brushed using a nylon straw brush containing SiCgrains. Examination of the brushed inserts in a light optical microscopeshowed that the outermost, thin TiN-layer and some of the Al₂O₃-layerhad been brushed away along the very cutting edge, leaving there asmooth Al₂O₃-surface. Coating thickness measurements on cross sectioned,brushed inserts showed that the outermost TiN-layer and roughly half theAl₂O₃-layer had been removed along the edge line.

Inserts from D and C were tested in face milling of grey cast ironcylinder heads.

Operation: Face milling - roughing Work-piece: Cylinder head Material:Pearlitic grey cast iron, alloyed, Cutting speed: 116 m/min Feedrate/tooth: 0.32 μm/rev. Depth of cut: 1.5-2 μm Insert-style: TNEF1204AN-CA Note: Wet, 13 teeth, unstable tendencies Results: Tool-life,number of component per edge set Grade D: (invention) 685 Grade C:(prior art) 475 Tool-life criterion was edge break-out on the work pieeedue to chipping and high flank wear of the edges.

EXAMPLE 3

E. Cemented carbide milling inserts in accordance with the invention,identical to the inserts described in D (Example 2), except for that thecoating not was brushed.

Inserts from D and E were tested in face milling of grey cast ironcylinder heads.

Operation: Face milling - roughing Work-piece: Cylinder head Material:Pearlitic grey cast iron, alloyed, cutting speed: 116 m/min Feedrate/tooth: 0.32 μm/rev. Depth of cut: 1.5-2 μm Insert-style: TNEF1204AN-CA Note: Wet, 13 teeth, unstable tendencies Results: Tool-life,number of component per edge set Grade D: (invention) 685 Grade E:(outside invention) 570 Tool-life criterion was edge break-out on thework piece due to chipping and high flank wear of the edge.

F. Cemented carbide milling inserts in accordance with the inventionwith the composition 7.6 wt. % Co, 1.25 wt. % TaC, 0.30 wt. % NbC andbalance WC with a grain size in average of 1.79 μm, with a binder phasealloyed with W corresponding to a CW-ratio of 0.86 were coated with a0.5 μm equiaxed TiC_(0.05)N_(0.95)-layer (with a high nitrogen contentcorresponding to an estimated C/N-ratio of 0.05) followed by a 2.7 μmthick TiC_(0.54)N_(0.46)-layer, with columnar grains by usingMTCVD-technique (temperature 850-885° C. and CH₃CN as thecarbon/nitrogen source). In subsequent steps during the same coatingcycle, a 1.2 μm thick layer of Al₂O₃ was deposited using a temperature970° C. and a concentration of H₂S dopant of 0.4% as disclosed in U.S.Pat. No. 5,674,564. A thin (0.8 μm) layer of TiN was deposited on topaccording to known CVD-technique. XRD-measurement showed that theAl₂O₃-layer consisted of 100% κ-phase.

The coated inserts were brushed using a nylon straw brush containing SiCgrains. Examination of the brushed inserts in a light optical microscopeshowed that the outermost, thin TiN-layer and some of the Al₂O₃-layerhad been brushed away along the very cutting edge, leaving there asmooth Al₂O₃-surface. Coating thickness measurements on cross sectioned,brushed inserts showed that the outermost TiN-layer and roughly half theAl₂O₃-layer had been removed along the edge line.

G. Commercial cemented carbide milling inserts with the composition of 8wt-% Co, 0.1 wt-% TiC, 1.7 wt-% TaC, 0.1 wt-% NbC, and balance WC andCW-ratio of 0.86. The WC-grain size was 1.74 μm. The inserts were coatedwith a 0.5 μm TiN-layer followed by a 1.5 μm thick TiC-layer and finallyfollowed by a 0.5 μm TiN-layer.

H. Commercial cemented carbide cutting inserts with the composition of 8wt. % Co, 0.1 wt. % TiC, 1.8 wt. % TaC, 0.1 wt. % NbC and balance WC,CW-ratio of 0.86 and WC-grain size 1.71 μm were coated with a 5 μmTiAlN-layer deposited by PVD-technique.

Inserts from F, G and H were tested in face milling of an alloyedpearlitic grey cast iron cylinder head.

Operation: Face milling - roughing Work-piece: Cylinder head Material:Pearlitic grey cast iron, alloyed, Cutting speed: 116 m/min Feedrate/tooth: 0.32 μm/rev. Depth of cut: 2 μm Insert-style: TNEF 1204ANNote: Wet, single tooth milling Results: Tool-life number of passes peredge Grade F: (invention) 78 Grade G: (prior art) 60 Grade H: (priorart) 58 Tool-life criterion was chipping and edge fractures of theedges.

EXAMPLE 5

I. Cemented carbide milling inserts in accordance with the inventionwith the composition 7.6 wt. % Co, 1.25 wt. % TaC, 0.30 wt. % NbC andbalance WC with a grain size in average of 1.75 μm, with a binder phasealloyed with W corresponding to a CW-ratio of 0.90 were coated with a0.5 μm equiaxed TiC_(0.05)N_(0.95)-layer (with a high nitrogen contentcorresponding to an estimated C/N-ratio of 0.05) followed by a 2.7 μmthick TiC_(0.54)N_(0.46)-layer, with columnar grains by usingMTCVD-technique (temperature 850-885° C. and CH₃CN as thecarbon/nitrogen source). In subsequent steps during the same coatingcycle, a 1.7 μm thick layer of Al₂O₃ was deposited using a temperature970° C. and a concentration of H₂S dopant of 0.4% as disclosed in U.S.Pat. No. 5,674,564. A thin (0.7 μm) layer of TiN was deposited on topaccording to known CVD-technique. XRD-measurement showed that theAl₂O₃-layer consisted of 100% κ-phase.

The coated inserts were brushed using a nylon straw brush containing SiCgrains. Examination of the brushed inserts in a light optical microscopeshowed that the outermost, thin TiN-layer and some of the Al₂O₃-layerhad been brushed away along the very cutting edge, leaving there asmooth Al203-surface. Coating thickness measurements on cross sectioned,brushed inserts showed that the outermost TiN-layer and roughly half theAl₂O₃-layer had been removed along the edge line.

Inserts from I and G were tested in face milling of pearlitic grey castiron engine blocks.

Operation: Face milling - roughing Work-piece: Engine block Material:Pearlitic gray cast iron, un-alloyed Cutting speed: 106 m/min Feedrate/tooth: 0.20 μm/rev Depth of cut: 3 μm Insert-style: TNEF 1204ANNote: Wet milling, 56 teeth per set Results: Tool-life, number ofcomponents per set Grade I: (invention) 975 Grade G: (prior art) 700Tool-life criterion was edge break-out on the work piece due to chippingand high flank wear of the edges.

EXAMPLE 6

Inserts from I and B were tested in face milling of pearlitic nodularcast iron gearbox housing.

Operation: Face milling - roughing Work-piece: Gear box housing.Material: Pearlitic nodular cast iron, alloyed Cutting speed: 137 m/minFeed rate/tooth: 0.15 μm/rev. Depth of cut: 5 μm Insert-style: TNEF1204AN-CA Note: Wet milling, 20 teeth, unstable tendencies Results:Tool-life, minutes of tool life per edge set Grade I: (invention) 105Grade B: (prior art)   60 Tool-life criterion was crack formation andchipping of the edges.

EXAMPLE 7

Inserts from I and C were tested in face milling of nodular cast ironengine block component

Operation: Face milling - roughing Work-piece: Engine block, bearingpart Material: Nodular cast iron Cutting speed: 93 m/min Feedrate/tooth: 0.25 μm/rev Insert-style: TNEF 1204AN-CA Note: Wet milling,26 teeth Results: Tool-life, number of components per edge set Grade I:(invention) 38000 Grade C: (prior art) 20000 Tool-life criterion wasburr and spalling on the work piece.

1. A method of making a milling insert comprising a cemented carbide body and a coating wherein the WC-Co-based cemented carbide body comprises WC, 7.3-7.9 wt. % Co and 1.0-1.8 wt. % cubic carbides of Ta and Nb and a highly W-alloyed binder phase with a CW-ratio of 0.86-0.94, the method comprising the steps of: depositing by a CVD-method a first, innermost layer of TiC_(x)N_(y)O_(z) with x+y+z=1, y>x and z<0.2 having an equiaxed grain structure with a size <0.5 μm and a total thickness of 0.1-1.5 μm; depositing by a MTCVD-technique a layer of TiC_(x)N_(y) with x+y=1, x>0.3 and y>0.3 with a thickness of 1-4 μm having a columnar grain structure with an average diameter of <5 μm, wherein the MTCVD-technique uses acetonitrile as a source of carbon and nitrogen for forming a layer in a temperature range of 700-900° C.; depositing a layer of a smooth κ-Al₂O₃ with a thickness of 1-2.5 μm; and depositing an outer layer of TiN with a thickness of 0.5-1.0 μm.
 2. The method according to the claim 1, wherein the cemented carbide body contains 1.4-1.7 wt. % carbides of Ta and Nb.
 3. The method according to claim 1, further comprising the step of removing the outer layer of TiN along a cutting edge.
 4. The method of making a milling insert of claim 1, wherein the first, innermost layer of TiC_(x)N_(y)O_(z) has y>0.8 and z=0.
 5. The method of making a milling insert of claim 1, wherein the layer of TiC_(x)N_(y) has x≧0.5.
 6. A method of wet milling comprising the steps of: providing a cutting tool insert comprising a cemented carbide body comprising WC, 7.3-7.9 wt. % Co, 1.0-1.8 wt. % cubic carbides of Ta and Nb, and a highly W-alloyed binder phase with a CW-ratio of 0.86-0.94, a coating comprising a first, innermost layer of TiC_(x)N_(y)O_(z) with x+y+z=1, y>x and z<0.2 having an equiaxed grain structure with a size <0.5 μm and a total thickness of 0.1-1.5 μm, a layer of TiC_(x)N_(y) with x+y=1, x>0.3 and y>0.3 with a thickness of 1-4 μm having a columnar grain structure with an average diameter of <5 μm, a layer of a smooth, fine-grained, 0.5-2 μm κ-Al₂O₃ with a thickness of 1-2.5 μm, and an outer layer of TiN with a thickness of 0.5-1.0 μm; operating the cutting tool insert at a speed of 70-180 m/min; and feeding at a rate of 0.1-0.4 μm/tooth, wherein the wet milling is wet milling a cast iron, a compacted graphite iron or a nodular iron.
 7. The method of claim 6, wherein the cast iron is a grey cast iron.
 8. The method according to claim 1, wherein an average grain size of the WC is about 1.5-2.5 μm.
 9. The method according to claim 8, wherein the average grain size of the WC is about 1.8 μm.
 10. The method according to claim 1, wherein the thickness of the TiC_(x) N _(y) layer is 2 - 2.7 μm.
 11. The method according to claim 1, wherein the average diameter of the columnar grain structure of the TiC_(x) N _(y) layer is 0.1-2 μm.
 12. The method according to claim 1, wherein the thickness of the layer of κ-Al ₂ O ₃ is 1.2 - 1.7 μm.
 13. The method according to claim 1, wherein the outer layer of TiN is removed along a cutting edge.
 14. The method according to claim 13, wherein the layer of κ-Al ₂ O ₃ is at least partly removed along the cutting edge.
 15. The method according to claim 1, comprising wet-blasting the coating surface with a fine-grained alumina powder or brushing the edges with a brush.
 16. The method of claim 6, wherein the outer layer of TiN is removed along a cutting edge.
 17. The method of claim 16, wherein the layer of κ-Al ₂ O ₃ is at least partly removed along the cutting edge. 